1. Field of the Invention
The present invention is related generally to power supplies and more particularly to regulation of a D.C. output voltage in a multi-output switching power supply.
2. Description of the Prior Art
In some power supplies, multiple secondary windings on the same transformer are used to generate multiple outputs. For example, one secondary winding could have the appropriate number of turns to yield a +5 v output, while a second secondary winding could have a different number of turns to yield a +12 v output.
In addition to the ratio of primary-to-secondary windings, further control is normally required to ensure the output is held reasonably constant. This control usually is achieved in a switching power supply by returning a feedback from one of the outputs and using this to control the duty cycle of the transistor(s) driving the transformer primary. Only one of the multiple outputs can act as the source of the feedback signal and, because the other outputs will not always track accurately enough, additional regulation is required for the non-controlling outputs.
The common methods for providing this extra regulation for the outputs which do not have direct feedback to the source are by means of a linear series pass regulator or a switching buck regulator. Both circuits involve the use of a regulating element, usually a transistor. Since the output current flows through the regulating element, a substantial amount of power is dissipated. The regulating element and the associated heat sink must, therefore, be of sufficient size and durability to withstand the environment. This prior art approach is functional, but is undesirable because of the size and cost of the components required and the power wasted in the form of heat.
The use of a tranformer in a linear regulator circuit is known in the prior art. However, no usage of a transformer in a switching power supply is known and no usage of a transformer of the design of the present invention is known in either type of power supply.
The present invention relates to a novel circuit for providing the additional regulation required by the multiple outputs which is free of the above noted problems.